1. Field of the Invention
The present invention is broadly concerned improved single screw extruders including therein improved screw sections which give positive conveyance even with highly preconditioned, low viscosity materials, as well as enhanced specific mechanical energy (SME) values. The improved screw sections of the invention include substantially smooth and arcuate surfaces between adjacent flighting portions, with the absence of any substantial angular discontinuities or “corners.” The performance of the single screw extruders can be enhanced with the use of new dual-shaft preconditioners having independent drives for the shafts and the ability to independently control the rotational speed and/or direction of the preconditioner shafts.
2. Description of the Prior Art
Extrusion systems are commonly used in the production of human foods and animal feeds. Broadly speaking, there are two types of extrusion systems, namely single screw and twin screw systems. As the names imply, single screw extruders include an elongated barrel with only a single, elongated, helically flighted, axially rotatable screw assembly therein. In contrast, twin screw extruders have specially configured barrels with a pair of juxtaposed, helically flighted, axially rotatable and intercalated screw assemblies. It is also quite common to employ a preconditioning device upstream of a single or twin screw extruder, which serves to at least partially cook the starting materials and gelatinized the starch-bearing components thereof. Normally, higher levels of cook and gelatinization are desired inasmuch as this lessens cooking requirements in the downstream extruder, leading to higher quality products and increased throughputs.
Single screw extruders are substantially less complex and expensive, as compared with twin screw extruders. Accordingly, producers of commodity products such as certain types of pet and aquatic feeds tend to prefer a single screw extruder systems. A persistent problem with single screw systems is the fact that highly gelatinized incoming feed materials tend to run unevenly with frequent surges, due to the sticky nature and low viscosities of the feed material. The viscosities of highly gelatinized feeds are relatively low, causing the feeds to build up at the base of the screw assembly with decreased forward conveyance through the extruder barrel. Such effects are exaggerated with incoming feeds containing meats, fish meals, and other high-fat ingredients, or high starch content pet foods. One answer to this problem is to reduce the extent of gelatinization achieved in the upstream preconditioner; but this leads to lower quality final products and lessens system throughputs.
Conventional single screw extruder screw sections are illustrated in FIGS. 9 and 10. Referring first to FIG. 9, the screw section 20 includes a central, tubular screw body 22 as well as outwardly extending, helical flighting 24. The section 20 presents a longitudinal axis A, as well as a root diameter RD. The outwardly extending flighting 24 presents an outer periphery P, thus establishing a flight depth FD between the root diameter of the body 22 and periphery P. In addition, the flighting 24 and body 22 cooperatively define surfaces 26 between adjacent portions of the flighting 24. The surfaces 26 include substantially planar surface portions 28 and 30 extending downwardly from the periphery P to the body 22, as well as a substantially planar intermediate portion 32 interconnecting the portions 28 and 30. It will be observed that the intersections between the planar surfaces 28, 30, and 32 present a relatively sharp angular discontinuities 34 and 36, as well as first and second angles α and β. In this case, the angles α and β are different, namely angle α is 30° relative to vertical, and angle β is 15° relative to vertical.
FIG. 10 illustrates another type of conventional single screw extruder screw element 38 which is similar to section 20, and like reference numerals and characters have been used. The principal difference between the sections 20 and 38 is that in the latter case the angles α and β are the same, namely 15° relative to vertical.
As can be seen from the configuration of these prior art single screw extruder screw elements, the provision of relatively deep flighting with the noted planar surfaces between flighting portions and the discontinuities or “corners” in such surfaces tends to create pockets or dead zones for material. As such, the materials tend to build up at the regions between the flighting portions until a completely full condition is reached, whereupon the extruder tends to “blow” or surge. This instability is a significant problem, and creates unsatisfactory final products.
There is accordingly a need in the art for improved single screw extruder screw sections and assemblies which are capable of handling highly preconditioned, relatively low viscosity incoming products without surging and with positive, even conveyance of the materials through the extruder barrel.